Amperometric sensors can be used to determine a measured variable dependent upon the concentration of an analyte, such as the concentration or activity of the analyte or of a parameter, in particular, of a sum parameter that involves the concentration or activity of the analyte. The measuring medium can be a measuring fluid, e.g., a measuring liquid or a measuring gas. Typical analytes, whose concentration or activity or measured variables dependent upon them can be monitored using amperometric sensors, are, for example, gases such as oxygen, chlorine, carbon dioxide, hydrogen sulfide, ammonium, or nitrogen oxide.
Such sensors often have an amperometric measuring probe that is brought into contact with the measuring medium for example, by immersion in the measuring medium. The measuring probe can include a measuring circuit, in particular, an on-site electronics unit that forms one of the measured variable-dependent measurement signals and is equipped to emit the measurement signal, or a signal derived from it using an initial processing, via an interface to a higher-level data processing device. The measuring probe having the measuring circuit can be connected wirelessly or via a line to a remote measuring transducer, which further processes the measurement signal of the measuring probe and outputs it via a user interface or a higher-level unit connected to the measuring transducer for wireless or hardwired communication, e.g., a process controller or a programmable logic controller.
Amperometric measuring probes usually have a probe interior space separated from the measuring medium by a sensor membrane also designated herein as an electrolyte chamber as well as at least two, often also three, electrodes arranged within the probe interior space. The electrodes are connected in an electrically conductive manner to the measuring circuit.
One of the electrodes functions as the measuring or working electrode, an additional one as a counter electrode. The sensor membrane generally includes at least one functional layer functioning as a diffusion barrier through which the analyte is diffused from the measuring medium into the electrolyte chamber. The measuring circuit creates the measurement signal that represents the measured variable, e.g., the analyte concentration, on the basis of a current flowing through the electrolyte between the measuring electrode and the counter electrode. In many amperometric applications, the potential of the measuring electrode or the current flow between the measuring electrode and the counter electrode through the electrolyte is regulated using a third reference electrode, through which current does not flow.
An amperometric measuring probe is described in, for example, DE 10 2008 039465 A1. Accommodated inside the electrolyte chamber of the measuring probe that is sealed by the sensor membrane is a fluid electrolyte with which the two or three electrodes are in contact. One of these electrodes acts as a measuring electrode. It is integrated into a rod-shaped electrode body, which isolates the measuring electrode in relation to the electrolyte, with the exception of its end face. The electrode body extends into the electrolyte chamber, wherein the end face of the measuring electrode abuts the sensor membrane. In this manner, a thin electrolyte film forms between the sensor membrane and the, for this purpose, optionally roughened or textured end face of the measuring electrode. The intermediate, electrolyte-filled space thus formed between the end face of the measuring electrode and the membrane is here and in the following described as a measuring chamber.
The measuring probe has a first housing part designed in the form of a cap and a second housing part that forms a probe. The first housing part includes the sensor membrane and is designated as the membrane module or membrane cap. The first and the second housing parts are detachably connected to one another via a screw connection. The detachable connection allows for replacement of the first housing part by an identically designed housing part, and thus for a replacement of the membrane.
If there are gas bubbles in the fluid electrolyte that is contained within the probe interior, there is a risk that they could pass into the measuring chamber formed between the end face of the electrode body and the sensor membrane. In the operation of the measuring probe, this leads to an interference in the current flowing between the measuring electrodes, and thus in the measurement signal of the measuring probe. This can adversely affect measurement precision, or even make measurement using the measuring probe impossible.
Care must therefore be taken to insure that the inclusion of gas bubbles in the probe interior is avoided during manufacture of the measuring probe, or possibly during an exchange of the housing part containing the sensor membrane that takes place during maintenance. Amperometric measuring probes known in the prior art have, for this purpose, a pressure relief valve that closes the probe interior. The membrane cap is filled with electrolyte before connection to the probe shaft, wherein the electrolyte volume filled into the membrane cap is greater than the volume of the probe interior formed after the connection of the membrane cap to the probe shaft. During connection of the membrane cap to the second housing part that forms the probe shaft, the second housing part seals the membrane cap while forming the probe interior, electrolyte exiting from the electrolyte chamber via the pressure release valve. In this manner, it is ensured that the probe interior is fully filled with fluid electrolyte, so that no gas bubbles remain in the probe interior. In the event of a malfunction of the pressure relief valve because of damage to or deterioration of the valve, for example, it cannot be ensured that the probe interior is reliably sealed with respect to the measuring medium, and that an intrusion of measuring medium into the probe interior or an exit of electrolyte from the probe interior into the measuring medium is reliably prevented.